In our prior U.S. Pat. No. 4,741,732 Crankshaw et al.gamma., the disclosure of which is incorporated herein by reference, we described a drug infusion pump having a drive system including a drive wheel on the output shaft of a DC gear motor biased to engage a resilient drive ring on a drive wheel which transmits drive to a drive head engaged by a toothed drive belt. While the described drive system was acceptable for an experimental pump designed to establish the workability of the drug infusion control system described in the application, subsequent consideration of the drive system revealed that it had certain shortcomings which made it unsuitable for commercial application.
In particular, it was found that the drive system had a tendency to reverse the drive motor due to recoil caused by deformation of the syringe, flexing of the support structure, elasticity in the drive belt and slackness in the drive system generally. Although the controller was able to successfully monitor such backlash and thereby monitor the actual position of the syringe plunger, direct computation of pump speed and plunger position was found to be most desirable. Furthermore, the achievement of lower drive speeds without recoil as well as wide speed ranges was found to be substantially impossible to achieve using the described drive system.
While more positive drive systems for the delivery of a drug from a syringe have been proposed, for example, see U.S. Pat. No. 4,407,659, such systems suffer from a number of disadvantages, including the inability to conveniently disconnect the drive to enable rapid positioning of the syringe actuator as well as the inability to achieve low delivery rates in combination with high delivery rates. As has been explained in the prior patent referred to above, drug infusion control systems often require very rapid delivery rates in the early stages of the control delivery in combination with very low rates of drug delivery in the latter stages of the infusion. The prior art drive systems do not allow the achievement of such delivery regimens.
A wide range of controlled infusion pumps which are adapted to deliver drugs from a syringe are currently available and are widely use for a number of purposes in the medical field. Several of these pumps have a limited ability to "recognize" one or more standard syringe sizes (Atom 235 and Terufusion STC-521). Other pumps are highly programmable to accept a range of different standard syringes (Medfusion Systems--Model 1001, Vial Medical Programme 3 and Sky Electronics--PS2000). However, none of the programmable infusion pumps currently available are able to recognize when a non-standard syringe is fitted to the pump or have the ability for the operator to input data relating to a non-standard syringe on the basis of which the delivery of fluid from such a syringe may be accurately controlled. The danger flowing from the above inability is the possibility of inaccurate pumping in the event that a non-standard syringe is used. At least some of the above pumps will operate when a non-standard syringe is inserted on the assumption one of the programmed standard syringes has in fact been inserted. The dangers of this type of operation are clearly obvious.